Getting narrow bias strips for binding on the true bias grain is a problem with squirmy fabrics. Here's my solution. You'll need two 18-inch or 24-inch rulers and one 6-inch square ruler (or bigger) that has 45 degree markings. The straight rulers keep the squirmy fabric held down. You wouldn't need to do this kind of thing on a stable fabric.

1. Lay out the fabric in a single layer as square as possible with your cutting mat, lining the selvage up with the edge or one of the printed lines. Weigh or pin down the selvage so it won't move. With large pieces of fabric, it can be easier to pool most of the fabric on the edge of your table, and just pull out as much as you think you'll need.

2. On the fabric selvage lay out the narrow edge of both straight rulers about six inches apart.

3. Put the square ruler between the two rulers. Move the two straight rulers to enclose the square ruler. Square up the rulers so that the edge of the square ruler is at equal distances on the straight ruler.